[llvm] [offload] [test] Skip amdgcn/nvptx tests if detected arch is not built (PR #119006)

[Michał Górny via llvm-commits]

mgorny wrote:

> A potentially better alternative would be to define a CMake variable when a target is enabled, and then check if the variable exists and is true when enabling tests.

You mean in `plugins-nextgen/*/CMakeLists.txt`? I suppose I could even reuse the existing logic there.

https://github.com/llvm/llvm-project/pull/119006